Transmembrane Segments of Syntaxin Line the Fusion Pore of Ca ² +-Triggered Exocytosis

The fusion pore of regulated exocytosis is a channel that connects and spans the vesicle and plasma membranes. The molecular composition of this important intermediate structure of exocytosis is unknown. Here, we found that mutations of some residues within the transmembrane segment of syntaxin (Syx), a plasma membrane protein essential for exocytosis, altered neurotransmitter flux through fusion pores and altered pore conductance. The residues that influenced fusion-pore flux lay along one face of an alpha-helical model. Thus, the fusion pore is formed at least in part by a circular arrangement of 5 to 8 Syx transmembrane segments in the plasma membrane.